1,567 research outputs found
Atomistic Theory of Coherent Spin Transfer between Molecularly Bridged Quantum Dots
Time-resolved Faradary rotation experiments have demonstrated coherent
transfer of electron spin between CdSe colloidal quantum dots coupled by
conjugated molecules. We employ here a Green's function approach, using
semi-empirical tight-binding to treat the nanocrystal Hamiltonian and Extended
Huckel theory to treat the linking molecule Hamiltonian, to obtain the coherent
transfer probabilities from atomistic calculations, without the introduction of
any new parameters. Calculations on 1,4-dithiolbenzene and
1,4-dithiolcyclohexane linked nanocrystals agree qualitatively with experiment
and provide support for a previous transfer Hamiltonian model. We find a
striking dependence on the transfer probabilities as a function of nanocrystal
surface site attachment and linking molecule conformation. Additionally, we
predict quantum interference effects in the coherent transfer probabilities for
2,7-dithiolnaphthalene and 2,6-dithiolnaphthalene linking molecules. We suggest
possible experiments based on these results that would test the coherent,
through-molecule transfer mechanism.Comment: 12 pages, 9 figures. Submitted Phys. Rev.
Response to “Every ROSE has its thorns”
Abstract: Sharp et al. [1] raise a number of concerns about the development and communication of ROSES (RepOrting standards for Systematic Evidence Syntheses), and we welcome the opportunity to explain some of the underlying thinking behind development of the reporting standards for environmental evidence syntheses
Electrical activation and electron spin coherence of ultra low dose antimony implants in silicon
We implanted ultra low doses (2x10^11 cm-2) of 121Sb ions into isotopically
enriched 28Si and find high degrees of electrical activation and low levels of
dopant diffusion after rapid thermal annealing. Pulsed Electron Spin Resonance
shows that spin echo decay is sensitive to the dopant depths, and the interface
quality. At 5.2 K, a spin decoherence time, T2, of 0.3 ms is found for profiles
peaking 50 nm below a Si/SiO2 interface, increasing to 0.75 ms when the surface
is passivated with hydrogen. These measurements provide benchmark data for the
development of devices in which quantum information is encoded in donor
electron spins
Quantum phases of dipolar rotors on two-dimensional lattices
The quantum phase transitions of dipoles confined to the vertices of two
dimensional (2D) lattices of square and triangular geometry is studied using
path integral ground state quantum Monte Carlo (PIGS). We analyze the phase
diagram as a function of the strength of both the dipolar interaction and a
transverse electric field. The study reveals the existence of a class of
orientational phases of quantum dipolar rotors whose properties are determined
by the ratios between the strength anisotropic dipole-dipole interaction, the
strength of the applied transverse field, and the rotational constant. For the
triangular lattice, the generic orientationally disordered phase found at zero
and weak values of both dipolar interaction strength and applied field, is
found to show a transition to a phase characterized by net polarization in the
lattice plane as the strength of the dipole-dipole interaction is increased,
independent of the strength of the applied transverse field, in addition to the
expected transition to a transverse polarized phase as the electric field
strength increases. The square lattice is also found to exhibit a transition
from a disordered phase to an ordered phase as the dipole-dipole interaction
strength is increased, as well as the expected transition to a transverse
polarized phase as the electric field strength increases. In contrast to the
situation with a triangular lattice, on square lattices the ordered phase at
high dipole-dipole interaction strength possesses a striped ordering. The
properties of these quantum dipolar rotor phases are dominated by the
anisotropy of the interaction and provide useful models for developing quantum
phases beyond the well-known paradigms of spin Hamiltonian models, realizing in
particular a novel physical realization of a quantum rotor-like Hamiltonian
that possesses an anisotropic long range interaction.Comment: Updated credit line and changed line spacin
LINVIEW: Incremental View Maintenance for Complex Analytical Queries
Many analytics tasks and machine learning problems can be naturally expressed
by iterative linear algebra programs. In this paper, we study the incremental
view maintenance problem for such complex analytical queries. We develop a
framework, called LINVIEW, for capturing deltas of linear algebra programs and
understanding their computational cost. Linear algebra operations tend to cause
an avalanche effect where even very local changes to the input matrices spread
out and infect all of the intermediate results and the final view, causing
incremental view maintenance to lose its performance benefit over
re-evaluation. We develop techniques based on matrix factorizations to contain
such epidemics of change. As a consequence, our techniques make incremental
view maintenance of linear algebra practical and usually substantially cheaper
than re-evaluation. We show, both analytically and experimentally, the
usefulness of these techniques when applied to standard analytics tasks. Our
evaluation demonstrates the efficiency of LINVIEW in generating parallel
incremental programs that outperform re-evaluation techniques by more than an
order of magnitude.Comment: 14 pages, SIGMO
Analytic, Group-Theoretic Density Profiles for Confined, Correlated N-Body Systems
Confined quantum systems involving identical interacting particles are to
be found in many areas of physics, including condensed matter, atomic and
chemical physics. A beyond-mean-field perturbation method that is applicable,
in principle, to weakly, intermediate, and strongly-interacting systems has
been set forth by the authors in a previous series of papers. Dimensional
perturbation theory was used, and in conjunction with group theory, an analytic
beyond-mean-field correlated wave function at lowest order for a system under
spherical confinement with a general two-body interaction was derived. In the
present paper, we use this analytic wave function to derive the corresponding
lowest-order, analytic density profile and apply it to the example of a
Bose-Einstein condensate.Comment: 15 pages, 2 figures, accepted by Physics Review A. This document was
submitted after responding to a reviewer's comment
High-quality variational wave functions for small 4He clusters
We report a variational calculation of ground state energies and radii for
4He_N droplets (3 \leq N \leq 40), using the atom-atom interaction HFD-B(HE).
The trial wave function has a simple structure, combining two- and three-body
correlation functions coming from a translationally invariant
configuration-interaction description, and Jastrow-type short-range
correlations. The calculated ground state energies differ by around 2% from the
diffusion Monte Carlo results.Comment: 5 pages, 1 ps figure, REVTeX, submitted to Phys. Rev.
Entangling flux qubits with a bipolar dynamic inductance
We propose a scheme to implement variable coupling between two flux qubits
using the screening current response of a dc Superconducting QUantum
Interference Device (SQUID). The coupling strength is adjusted by the current
bias applied to the SQUID and can be varied continuously from positive to
negative values, allowing cancellation of the direct mutual inductance between
the qubits. We show that this variable coupling scheme permits efficient
realization of universal quantum logic. The same SQUID can be used to determine
the flux states of the qubits.Comment: 4 pages, 4 figure
Efficient energy transfer in light-harvesting systems, I: optimal temperature, reorganization energy, and spatial-temporal correlations
Understanding the mechanisms of efficient and robust energy transfer in
light-harvesting systems provides new insights for the optimal design of
artificial systems. In this paper, we use the Fenna-Matthews-Olson (FMO)
protein complex and phycocyanin 645 (PC 645) to explore the general dependence
on physical parameters that help maximize the efficiency and maintain its
stability. With the Haken-Strobl model, the maximal energy transfer efficiency
(ETE) is achieved under an intermediate optimal value of dephasing rate. To
avoid the infinite temperature assumption in the Haken-Strobl model and the
failure of the Redfield equation in predicting the Forster rate behavior, we
use the generalized Bloch-Redfield (GBR) equation approach to correctly
describe dissipative exciton dynamics and find that maximal ETE can be achieved
under various physical conditions, including temperature, reorganization
energy, and spatial-temporal correlations in noise. We also identify regimes of
reorganization energy where the ETE changes monotonically with temperature or
spatial correlation and therefore cannot be optimized with respect to these two
variables
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